1. Field of the Invention
The present invention is directed to a process of manufacturing cones of drilling bits which have hard cutter inserts. More particularly, the present invention is directed to a process of laser hardfacing the outer shell surfaces of roller cone bits of the type which have hard tungsten carbide or like cutter inserts.
2. Brief Description of the Prior Art
One important type of rotary drill bit used for subterranean drilling includes cutter cones which have hard tungsten carbide or like cutter inserts. Usually such cutter cones are rotatably mounted on journal legs of the drill bit so as to rotate as the drill bit is rotated. The drill bit may be rotated from the surface, or by a "downhole" motor. The tungsten carbide or like hard cutter inserts of cutter cones are pressed into insert holes formed in the external surface of the cutter cones. These tungsten carbide inserts bear against the rock formation at the bottom of the hole, crushing and chipping the rock as drilling proceeds.
Because rock drilling is a technically very demanding service, and because failure of a drilling bit can cause very costly interruption in the drilling process, the construction of rock bits must be very rugged. Usually the cones of the drilling bit are made of forged alloy steel, although powder metallurgy and related cones have also been described in patent and technical literature. Bearing surfaces are located within the interior of the cones to enable rotatable mounting to the journal leg. An effective seal must be provided between the rotating cone and the journal leg so as to prevent escape of lubricating grease from the bearings and to prevent entry of drilling fluid and other foreign matter in the bearing.
The steel body of the cone itself must be sufficiently ductile and tough so as to avoid fracture or shattering. Certain parts of the interior of the cone, particularly the ball bearing races, must be quite hard in order to provide sufficiently long bearing life. The exterior of the cutter cone ideally should also be quite hard and abrasion resistant so as to avoid rapid wear due to its exposure to the formation and the highly abrasive and erosive action of the drilling fluid.
The tungsten carbide or other hard inserts in the roller cones must be held sufficiently strongly so as to prevent premature loss. The inserts must also be prevented from rotating in the insert holes because rotation in the insert hole leads to decreased drilling efficiency and eventually to loss of the insert.
In view of the economic importance of subterranean drilling for oil and other minerals, the prior art has developed a variety of technological approaches to more or less satisfy the above-summarized requirements.
In accordance with one basic approach, the forged steel cone body is made of a "carburizable" low carbon steel which has sufficient ductility and toughness to be adequately resistant to fracture. Certain parts of the interior of the cone, such as the bearing races, may be carburized to increase their hardness. The exterior of the cone however does not have a hardened case. A hardened exterior case would prevent insert holes from being easily drilled and is therefore not practical because of manufacturing limitations.
In light of the manufacturing difficulty of drilling holes through a hard case, most roller cones have an exterior shell surface which is not carburized and have a surface hardness of only approximately 40 Rockwell C (Rc) hardness units. Whereas the alloy steel of these cones is adequately ductile and tough, lack of external shell surface hardness and abrasion resistance results in relatively rapid wear and erosion of the cone shell during drilling, often resulting in loss of tungsten carbide inserts and inadequate bit performance.
One approach to increasing the hardness of the cone shell steel around inserts is covered by U.S. Pat. No. 4,708,752, assigned to the same assignee as the present invention, cited here for cross-reference. In this application, a medium to high carbon steel body of a roller cone bit is machined to final dimensions and then etched black with a paint or phosphate etch. Holes are drilled for inserts in the body and then the inserts are press fitted into the body. The inserts themselves are shiny and reflective whereas the black etched (or painted) body is rendered absorbent to laser light. The body is subjected to laser treatment using, for example, a carbon-dioxide laser which raises the surface of the blackened steel body to above austenitizing temperature. Rapid self-quenching results in a hard martensitic layer forming on the external surface with a hardness of 57 to 60 Rockwell C units. This hard layer is present in the steel even in the immediate vicinity of the inserts. The inserts themselves have shiny heat reflective surfaces and therefore do not absorb laser light and are consequently not affected.
Because of the variety of rock formations being drilled and the variation in loads/speeds to which the bit is subjected, methods other than hardening of steel need to be investigated to minimize erosion and abrasive wear. Hardfacing of steel by manual oxyacetylene methods has been a traditional approach applied on different downhole tools to reduce steel wear. A number of patents have issued related to hardfacing steel components and hardfacing mixtures, all used to reduce wear and erosion. However, these methods and teachings cannot be applied toward hardfacing the cone shell of rock bits around the inserts, as will be discussed.
U.S. Pat. No. 3,989,554, for example, teaches the use of a composite hardfacing of air hardening steel and particles of tungsten carbide, used to increase the wear resistance of manufactured articles such as tool joints. The outer surfaces of bit legs for shirttails have been hardfaced with atomic hydrogen torches, as taught in
U.S. Pat. No. 3,158,214. However, all of these methods employ heat sources that cause some extent of melting and dilution of the substrate to which these are applied. Consequently, if attempts were made to hardface a finished cone with inserts, melting of the substrate would destroy the dimensional aspects of the cone and also damage the interference fit between inserts-insert holes that essentially retain inserts in the cone-shell during drilling
Limitations in manufacturing also make it impossible to first conventionally hardface the exterior of a cone and then attempt to drill through this hard coating to press inserts into the cone.
As is apparent from the foregoing, there is still a substantial need in the prior art for a process for substantially hardfacing, in an economically feasible manner, the exterior shell surface of hard insert bearing roller cones in the immediate vicinity of inserts. The present invention provides such a process.